1998 Olds Bravada P0135 Code: Oxygen Sensor Heater Circuit Issue and Bosch Sensor Problems

Did you install a Bosch oxygen sensor during the replacement?

Yes, I used a Bosch sensor. However, I haven't yet inspected the wiring for any breaks or damage.

Even with a Bosch sensor, the issue is likely still related to the oxygen sensor itself—especially if it has faulty heater elements.

Why are Bosch sensors often suspected in these cases?

Many Bosch oxygen sensors have heater elements with incorrect resistance values. The Powertrain Control Module (PCM) detects this mismatch and disables the heater circuit to protect itself, which results in a persistent P0135 code—even if the sensor appears new. This is a known issue where inferior materials or design flaws cause incompatibility, not just open circuits. There are numerous forum posts confirming this problem with Bosch sensors.

I'm relatively new to car diagnostics—what's the best way to test an oxygen sensor?

The sensor should be tested using an oscilloscope. Any other method, like a digital multimeter (DVOM), only checks for stuck bias voltage and cannot capture the rapid signal fluctuations of a healthy O2 sensor. Since your P0135 code is present, it strongly suggests a faulty heater element—especially in new Bosch sensors, which often have incorrect resistance values. These sensors won't show an open circuit; instead, they're incompatible with the PCM's driver.

I'll try replacing the sensor again. If that doesn’t resolve it, I’ll investigate for broken or corroded wiring in the harness.

That’s a reasonable approach—though it's less common than a defective oxygen sensor; wiring issues are usually secondary.

Thanks for the clarification—it helps a lot!